Semiconductor device

ABSTRACT

A semiconductor device of SOI structure with improved ESD resistance is provided. In an I/O protection circuit of a semiconductor device of SOI structure, each unit channel width resistance value of drain resistance of a plurality of NMOS transistors, each being provided in parallel to an external terminal by a reverse bias connection, is set to have an HBM surge resistance voltage equal to that in a forward bias connection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device comprising anMOS transistor formed on an SOI (silicon(semiconductor)-on-insulator)substrate and, in particular, to its I/O protection function.

2. Description of the Background Art

In MOS transistors formed on SOI substrates in which a silicon thin filmis formed on an insulating substrate (referred to as “SOI device” insome cases), its source and drain regions reach the insulating substrateso that each junction capacity is reduced, permitting high speed and lowdissipation power operation.

Specifically, in MOS transistors formed on normal bulk siliconsubstrates (referring to as “substrate device” in some cases), therespective junction capacity of drain and source regions is increased atlow voltages and its performance (particularly, operation speed) isextremely lowered. On the other hand, since SOI devices have lesscomponents for the respective junction capacity of drain and sourceregions, so that they have little performance degradation, permittinghigh speed and low dissipation power operation.

As described, since SOI devices provide high speed and low dissipationpower operation, they are highly anticipated to be utilized as devicesfor portable apparatuses.

Although SOI devices are expected to be used as devices for low-voltageoperation, they have poor ESD (Electro Static Discharge) resistance inI/O protection circuits. The reason for being called I/O protectioncircuit is that a surge voltage can be applied from output terminals aswell as input terminals.

FIG. 23 is a sectional view of an SOI structure. As shown in the figure,a buried oxide film 2 is formed on a semiconductor substrate 1, and anSOI layer 3 is formed on the oxide film 2. The structure comprising thesemiconductor substrate 1, the buried oxide film 2, and the SOI layer 3,is called SOI substrate. A gate oxide film 4 is selectively formed onthe SOI layer 3, and a gate electrode 5 is formed on the gate oxide film4. The SOI layer 3 underlying the gate oxide film 4 serves as a channelregion 8, and regions of the SOI layer 3 adjacent to the channel region8 serve as a drain region 6 and a source region 7.

In the above SOI structure, heat generated when applied a surge voltageis accumulated by the presence of the buried oxide film 2 whose thermalconductivity is poor (about one-tenth of that of silicon). It istherefore liable to cause a thermal runaway or 2nd breakdown, resultingin permanent breakage of semiconductor devices on SOI substrates.

FIG. 24 is a graph diagram showing a process of a thermal runaway. Thisfigure shows a process of a thermal runaway at the time of reverse biasconnection, e.g., when an input voltage (surge voltage SV) is applied toa drain of an NMOS transistor Q1 whose source and gate are grounded asshown in FIG. 25. The thermal runaway process of FIG. 24 comprisessubprocesses P1 to P5.

When a surge voltage SV far beyond ordinary ones is applied to the drainof the NMOS transistor Q1 in FIG. 25, its drain voltage rises rapidly(P1) and reaches a breakdown induced voltage. Then, the transistor Q1causes an avalanche breakdown so that the current begins to flow,lowering to a holding voltage temporarily (P2). Thereafter, it starts torise again (P3) and, when it reaches a thermal breakdown voltage,portions of the transistor Q1 become a melted state so that theresistance value between the source and drain is rapidly lowered,causing a rapid voltage drop (P4). The flow of current concentrates onthe transistor Q1 that has caused such a rapid drop of resistance valuebetween the source and drain, that is, a positive feedback is effected(P5). As a result, the transistor Q1 is completely broken. For instance,as shown in FIG. 26, a large defect 10 due to the thermal breakdownoccurs in the gate electrode 5, thereby making the transistor operationimpossible.

Generally, in cases where, as an I/O protection circuit on an SOIsubstrate, NMOS transistors are provided by a reverse bias, NMOStransistors having a channel width W are connected in parallel betweenan input (voltage) IN and a ground level as shown in FIG. 27. In thecase of FIG. 27, six NMOS transistors T1 to T6 whose gate is groundedare provided in parallel between an input IN and a ground level as shownin FIG. 28. The NMOS transistor T1 comprises a gate electrode 51, adrain region 61, and a source region 71. The NMOS transistor T2comprises a gate electrode 52, a drain region 61, and a source region72. The NMOS transistor T3 comprises a gate electrode 53, a drain region62, and a source region 72. The NMOS transistor T4 comprises a gateelectrode 54, a drain region 62, and a source region 73. The NMOStransistor T5 comprises a gate electrode 55, a drain region 63, and asource region 73. The NMOS transistor T6 comprises a gate electrode 56,a drain region 63, and a source region 74. The input voltage IN as asurge voltage is input from an external input terminal or an externaloutput terminal.

Providing the six NMOS transistors T1 to T6 in parallel between theinput IN and the ground level as an I/O protection circuit on an SOIsubstrate, enables to distribute the current into the transistors T1 toT6 when the current flows between the input IN and the ground level.

If, however, one of the NMOS transistors T1 to T6 causes an avalanchebreakdown and reaches a thermal breakdown voltage, a resistance valuebetween the source and drain of such a transistor is rapidly lowered. Asshown in a subprocess P5 in FIG. 24, the flow of current concentrates onsuch a transistor that has reached a thermal breakdown voltage, failingto suppress this transistor from being broken.

Thus, with the I/O protection circuit utilizing the NMOS transistors ofthe conventional SOI structure, even if, in order to improve a surgeresistance, a plurality of NMOS transistors in parallel connectionconstitute an I/O protection circuit as shown in FIG. 27, expectedimprovement in ESD resistance cannot be accomplished.

SUMMARY OF THE INVENTON

According to a first aspect of the present invention, a semiconductordevice formed on an SOI substrate has an I/O protection circuit portionincluding at least one first MOS transistor connected to an externalterminal by a forward bias, and a plurality of second MOS transistorsconnected in parallel to the external terminal by a reverse bias. Inthis semiconductor device, a resistance value of each drain resistanceof the plurality of second MOS transistors is set so that an ESD(Electro Static Discharge) resistance of the plurality of second MOStransistors is approximately equal to or greater than that of the atleast one first MOS transistor.

According to a second aspect of the invention, the semiconductor deviceof the first aspect further comprises an internal circuit portion thatperforms signal processing based on signals from the external terminal.In this semiconductor device, the internal circuit includes an MOStransistor for internal circuit having a conductivity type identicalwith that of the plurality of second MOS transistors, and the MOStransistor for internal circuit has a drain resistance whose resistancevalue is smaller than that of each drain resistance of the plurality ofsecond MOS transistors.

According to a third aspect of the invention, in the semiconductordevice of the second aspect, a plurality of first silicide layers arerespectively provided on drain regions of the plurality of second MOStransistors, a second silicide layer is provided on a drain region ofthe MOS transistor for internal circuit, and the first silicide layer isthinner in thickness than the second silicide layer.

According to a fourth aspect of the invention, in the semiconductordevice of the first aspect, the conductivity type of the at least onefirst MOS transistor and the plurality of second MOS transistors isn-type.

According to a fifth aspect of the invention, in the semiconductordevice of the fourth aspect, each resistance value of the plurality ofsecond MOS transistors is not less than 30 Ω per 1 μm of channel width.

According to a sixth aspect of the invention, a semiconductor deviceformed on an SOI substrate has an I/O protection circuit portionconnecting an MOS transistor for I/O protection to an external terminal,and an internal circuit comprising an MOS transistor for internalcircuit to perform a prescribed signal processing therein. In thissemiconductor device, a drain region of the MOS transistor for I/Oprotection has first regions upon which no silicide layer is formed, anda second region upon which a silicide layer is formed and a drain regionof the MOS transistor for internal circuit has a third region upon whicha silicide layer is formed; the first regions being higher in impurityconcentration than the third region.

According to a seventh aspect of the invention, the semiconductor deviceof the sixth aspect is characterized in that the second region is lowerin impurity concentration than the first regions.

According to an eighth aspect of the invention, a semiconductor deviceformed on an SOI substrate has an I/O protection circuit portionconnecting a PMOS transistor for I/O protection to an external terminal.In this semiconductor device, the channel region property of said PMOStransistor is set to improve an ESD resistance of the PMOS transistorwithout affecting an integration degree.

According to a ninth aspect of the invention, the semiconductor deviceof the eighth aspect is characterized in that the I/O protection circuitportion further includes an NMOS transistor for I/O protection, and achannel length of the PMOS transistor for I/O protection is shorter thana channel length of the NMOS transistor for I/O protection.

According to a tenth aspect of the invention, the semiconductor deviceof the eighth aspect further comprises an internal circuit portionincluding a PMOS transistor for internal circuit to perform signalprocessing therein. In this semiconductor device, portions adjacent to achannel region in a drain region of the PMOS transistor for I/Oprotection is higher in impurity concentration than portions adjacent toa channel region in a drain region of the PMOS transistor for internalcircuit.

According to an eleventh aspect of the invention, the semiconductordevice of the eight aspect further comprises an internal circuit portionhaving a PMOS transistor for internal circuit to perform signalprocessing therein. In this semiconductor device, the PMOS transistorfor I/O protection is shorter in channel length than the PMOS transistorfor internal circuit.

According to a twelfth aspect of the invention, a semiconductor deviceformed on an SOI substrate has an I/O protection circuit correspondingto an external terminal. The I/O protection circuit has only NMOStransistors connected between a power supply and a ground level.

According to a thirteenth aspect of the invention, a semiconductordevice formed on an SOI substrate has an I/O protection circuitcorresponding to an external terminal, and further includes a powersupply wire connected to a power supply in common and a ground wireconnected to a ground level in common. In this semiconductor device, theI/O protection circuit includes an NMOS transistor in a diode connectionbetween the power supply wire and the ground wire, and an MOS transistorfor I/O protection provided between the external terminal and at leastone of the power supply wire and the ground wire.

According to a fourteenth aspect of the invention, the semiconductordevice of the thirteenth aspect further comprises a capacitor providedbetween the power supply wire and the ground wire of the I/O protectioncircuit portion.

In the semiconductor device of the first aspect, in order that the ESDresistance due to a plurality of second MOS transistors connected by areverse bias is approximately equal to or greater than the ESDresistance due to at least one of first MOS transistors connected by aforward bias, the resistance value of drain resistance of the second MOStransistors is set respectively. It is therefore possible to obtain I/Oprotection circuits that exhibit excellent ESD resistance in bothforward and reverse connections of the MOS transistors.

In the semiconductor device of the second aspect, the MOS transistor forinternal circuit has a smaller resistance value of drain resistance thaneach drain resistance of the plurality of second MOS transistors. Thisprevents the drive capability of the MOS transistor for internal circuitfrom lowering than necessary.

In the semiconductor device of the third aspect, by decreasing the filmthickness of a plurality of first silicide layers in the MOS transistorfor I/O protection than that of the second silicide layer in the MOStransistor for internal circuit, the drain resistance of the MOStransistor for I/O protection can be increased than that of the MOStransistor for internal circuit. It is therefore possible to increasethe resistance value of drain resistance due to the second MOStransistors in the I/O protection circuit so as to have an ESDresistance approximately equal to that due to at least one fist MOStransistor, and also to decrease the resistance value of drainresistance of the MOS transistor for internal circuit so as to obtain adesired drive capability.

In the semiconductor device of the fourth aspect, the conductivity typeof at least one first MOS transistor and a plurality of second MOStransistors is n-type. NMOS transistors produce a snap back phenomenonthat at the time of reverse bias connection, its drain voltage reaches abreakdown induced voltage to cause an avalanche breakdown so that theresistance value between the source and drain is lowered and the drainvoltage is then lowered. This results in the problem that the flow ofcurrent concentrates on an NMOS transistor that has caused an avalanchebreakdown for the first time to reduce the resistance value between thesource and drain, leading to a thermal breakdown. However, by increasingthe each resistance value of drain resistance of the plurality of secondMOS transistor so as to have an ESD resistance approximately equal tothat of at least one first MOS transistor, it is possible to suppress areduction in resistance value between the source and drain because ofthe snap back phenomenon, thereby avoiding the above problem.

In the semiconductor device of the fifth aspect, by setting eachresistance value of a plurality of second NMOS transistor per 1 μm ofchannel width to 30 Ω or more, it is possible to obtain an ESDresistance approximately equal to that of at least one first MOStransistor connected by a forward bias.

In the semiconductor device of the sixth aspect, the impurityconcentration of the first region upon which no silicide layer is formedin the MOS transistor for I/O protection, is higher than that of thethird region upon which the silicide layer is formed in the MOStransistor for internal circuit. By decreasing the resistance value ofthe first region, transistors for I/O protection circuit having a highdrive capability can be obtained.

In the semiconductor device of the seventh aspect, the impurityconcentration of a second region upon which the silicide layer is formedin the MOS transistor for I/O protection, is lower than that of thefirst region upon which no silicdie layer is formed, thus exerting noharmful effect on the silicide layer formed in the first region,irrespective of the impurity concentration of the second region.

In the semiconductor device of the eighth aspect, the channel regionproperty of the PMOS transistor is set to improve the ESD resistancewithout affecting the integration degree of the PMOS transistors for I/Oprotection. This enables to improve the ESD resistance of PMOStransistors that are generally inferior to NMOS transistors in ESDresistance, thus reducing a difference in ESD resistance between thePMOS transistors for I/O protection and the NMOS transistors for I/Oprotection. This permits the I/O protection circuits whose ESDresistance does not deteriorate even if PMOS transistors are used.

In the semiconductor device of the ninth aspect, the channel length ofthe PMOS transistor for I/O protection is shorter than that of the NMOStransistor for I/O protection. This enables to lower the breakdowninduced voltage so that the ESD resistance is improved without affectingthe integration degree.

In the semiconductor device of the tenth aspect, the impurityconcentration of portions adjacent to the channel region in the drainregion of the PMOS transistor for I/O protection is higher than that ofportions adjacent to the channel region in the drain region of the PMOStransistor for internal circuit. This enables to lower the breakdowninduced voltage so that the ESD resistance is improved without affectingthe integration degree.

In the semiconductor device of the eleventh aspect, the channel lengthof the PMOS transistor for internal circuit is shorter than that of thePMOS transistor for I/O protection. This enables to lower the breakdowninduced voltage so that the ESD resistance is improved without affectingthe integration degree.

In the I/O protection circuit of the semiconductor device of the twelfthaspect, only the NMOS transistors are connected between a power supplyand a ground level. Since the NMOS transistors are superior to the PMOStransistors in ESD resistance, it is possible to obtain I/O protectioncircuits excellent in ESD resistance.

The I/O protection circuit of the semiconductor device of the thirteenthaspect includes the NMOS transistor provided between the power supplywire and the ground wire by a diode connection, and the MOS transistorfor I/O protection circuit provided between the external terminal and atleast either of the power supply wire and the ground wire. Therefore,when a surge voltage is applied to the external terminal, the dischargecurrent flows through a current path connected between the externalterminal, the MOS transistor for I/O protection, one of the power supplywire and the ground wire connected the MOS transistor for I/Oprotection, the NMOS transistor and the other of the power supply wireand the ground wire. As a result, the surge voltage is always dischargedthrough the NMOS transistor excellent in ESD resistance, making itpossible to obtain I/O protection circuits excellent in ESD resistanceon SOI substrates.

In the semiconductor device of the fourteenth aspect, the capacitor isadded between the power supply and the ground level in the I/Oprotection circuit. Therefore, a surge voltage can be charged by thecapacitor, thereby dispersing the surge voltage.

An object of the present invention is to provide a semiconductor deviceof SOI structure with improved ESD resistance.

These and other object, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph diagram showing a characteristic of a surge resistancevoltage of an NMOS transistor at the time of reverse bias connection;

FIG. 2 is a graph diagram showing a characteristic of a surge resistancevoltage of an NMOS transistor at the time of forward bias connection;

FIG. 3 is a circuit diagram showing an NMOS transistor in a forward biasconnection;

FIG. 4 is a diagram for explaining a thermal runaway process of an NMOStransistor in a reverse bias connection;

FIG. 5 is a diagram for explaining a thermal runaway process of an NMOStransistor in a forward bias connection;

FIG. 6 is a circuit diagram showing a structure for making a forwardbias connection utilizing a plurality of NMOS transistors;

FIG. 7 is a plan view showing a typical plan structure of an NMOStransistor used for an I/O protection circuit of a semiconductor deviceaccording to a first preferred embodiment of the present invention;

FIG. 8 is a sectional view showing a structure of an NMOS transistorused for an I/O protection circuit of the first preferred embodiment;

FIG. 9 is a sectional view showing a structure of an NMOS transistorused for an internal circuit portion of a second preferred embodiment ofthe present invention;

FIG. 10 is a sectional view showing a structure of an NMOS transistorused for an I/O protection circuit portion of the second preferredembodiment;

FIG. 11 is a sectional view showing another structure of an NMOStransistor used for an I/O protection circuit of the second preferredembodiment;

FIG. 12 is a plan view showing a plan structure of a PMOS transistor andan NMOS transistor used in a first aspect of a third preferredembodiment of the present invention;

FIG. 13 is a sectional view showing a structure of a PMOS transistorused for an internal circuit portion of a second aspect of the thirdpreferred embodiment;

FIG. 14 is a sectional view showing a structure of a PMOS transistorused for an I/O protection circuit portion of the second aspect of thethird preferred embodiment;

FIG. 15 is a sectional view showing a structure of a PMOS transistorused for an I/O protection circuit portion in a third aspect of thethird preferred embodiment;

FIG. 16 is a sectional view showing a structure of a PMOS transistorused for an I/O protection circuit portion in a fourth aspect of thethird preferred embodiment;

FIG. 17 is a circuit diagram showing a circuit structure of a firstaspect of an I/O protection circuit of a semiconductor device accordingto a fourth preferred embodiment of the present invention;

FIG. 18 is a circuit diagram showing a circuit structure of a secondaspect of an I/O protection circuit of a semiconductor device accordingto the fourth preferred embodiment;

FIG. 19 is a circuit diagram showing a circuit structure of a firstexample of a third aspect of an I/O protection circuit according to thefourth preferred embodiment;

FIG. 20 is a circuit diagram showing a circuit structure of a secondexample of the third aspect of an I/O protection circuit according tothe fourth preferred embodiment;

FIG. 21 is a circuit diagram showing a circuit structure of a thirdexample of the third aspect of an I/O protection circuit according tothe fourth preferred embodiment;

FIG. 22 is a circuit diagram showing a circuit structure of the thirdaspect of an I/O protection circuit according to the fourth preferredembodiment;

FIG. 23 is a sectional view of an SOI structure;

FIG. 24 is a diagram for explaining a thermal runaway process in a NMOStransistor in a reverse bias connection;

FIG. 25 is a circuit diagram showing an NMOS transistor in a reversebias connection;

FIG. 26 is a plan view showing a plan structure of an MOS transistorthat has been broken by a thermal runaway;

FIG. 27 is a plan view showing a plan structure of a plurality of NMOStransistors connected in parallel between an input and a ground; and

FIG. 28 is a circuit diagram showing a circuit structure in FIG. 27.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The First Preferred Embodiment

The Principle of The Invention

FIG. 1 is a graph diagram showing a characteristic of HBM (Human BodyModel) resistance of an NMOS transistor that is in a reverse biasconnection to an input IN obtained from an external terminal, as shownin FIG. 25. The graph of FIG. 1 is a plot of an HBM surge resistancevoltage to a drain resistance value (hereinafter referred to as unitchannel width resistance value) per 1 μm of channel width in an NMOStransistor whose channel width W is 400 μm and channel length L is 0.6μm.

The channel width W adopted for use herein means the total value of eachNMOS transistor when a plurality of NMOS transistors are provided inparallel between an input IN and a ground level by a reverse bias. Incases where six NMOS transistors T1 to T6 are used as shown in FIGS. 27and 28, each gate width of the respective NMOS transistors T1 to T6 is400/6 μm.

Referring to FIG. 1, at the time of reverse bias connection, the HBMsurge resistance voltage increases in proportion to the unit channelwidth resistance value in about 10-50 Ω range of the unit channel widthresistance value of drain resistance. Over 50 Ω, the HBM surgeresistance voltage stabilizes at about 3.0 KV.

FIG. 2 is a graph diagram showing the HBM resistance of an NMOStransistor Q2 connected to an input IN obtained from an externalterminal by a forward bias connection as shown in FIG. 3. The graph ofFIG. 2 is a plot of the HBM surge resistance voltage to the unit channelwidth resistance value of drain resistance in an NMOS transistor whosechannel width W is 400 μm and channel length L is 0.6 μm, as in FIG. 1.

As can be seen from FIG. 2, at the time of forward bias connection, theHBM surge resistance voltage stabilizes at about 1.5 KV, irrespective ofthe unit channel width resistance value of drain resistance.

Next, description will be given of the reason why at the time of reversebias connection, the HBM surge resistance voltage changes with the unitchannel width resistance value whereas at the time of forward biasconnection, the HBM surge resistance voltage stabilizes irrespective ofthe unit channel width resistance value.

At the time of reverse bias connection, an NMOS transistor is brokenthrough the process as shown in FIG. 24. However, since the thermalbreakdown voltage is lower than a breakdown induced voltage, even if thedrain voltage of the NMOS transistor is once lowered after reaching thebreakdown voltage to cause an avalanche breakdown, it readily leads tothe thermal breakdown voltage by an additional surge voltage SV. Thisresults in an element breakdown due to a thermal runaway as previouslymentioned.

Specifically, even though a plurality of NMOS transistors are connectedin parallel as shown in FIGS. 27 and 28, if part of which cause anavalanche breakdown, its resistance value is lowered at the same time,the drain voltage of other transistors causing no avalanche breakdownare also lowered, preventing said other transistors from leading to anavalanche breakdown. Then, the flow of current concentrates on thetransistors led to the avalanche breakdown. This causes a rise intemperature, readily leading to a thermal breakdown at a thermalbreakdown voltage lower than a breakdown induced voltage.

On the other hand, when a plurality of NMOS transistors are connected inparallel at high drain resistance values, even if part of which cause anavalanche breakdown, because of high drain resistance, the resistancevalue between the source and drain of the NMOS transistors led to theavalanche breakdown is not so lowered. Therefore, it is avoided that theflow of current concentrates on the transistors led to the avalanchebreakdown. The degree of temperature rise is low. Thus, a thermalbreakdown is suppressed until it reaches a thermal breakdown voltagehigher than a breakdown induced voltage.

In addition, since the thermal breakdown voltage is higher than thebreakdown induced voltage, all the transistors are to reach an avalanchebreakdown before a thermal breakdown phenomenon occurs. Thus, thecurrent can flow uniformly to a plurality of transistors, therebyincreasing the ESD resistance.

That is, relatively high value of each drain resistance of a pluralityof NMOS transistors provided in parallel between an input IN and aground level by a reverse bias connection, prevents the flow of currentfrom concentrating on the transistor that has caused an avalanchebreakdown. This enables to set a thermal breakdown voltage higher than abreakdown induced voltage as shown in the solid lines in FIG. 4, therebyimproving the ESD resistance. The broken lines of FIG. 4 show caseswhere a drain resistance is relatively low.

From the above reason, it is evident that the HBM resistance can beimproved by setting the unit channel width resistance of drainresistance to greater values at the time of reverse bias connection. Thegraph of FIG. 1 shows that the saturated value of 3.0 KV can be obtainedby setting the unit channel width resistance value of drain resistanceto 50 Ω or more. In other words, the unit channel width resistance valueof not less than 50 Ω permits a plurality of NMOS transistors to producea uniform discharge, allowing all the transistors to display their fullrange of discharge capability.

FIG. 5 is a graph diagram showing a discharge characteristic at the timeof forward bias connection shown in FIG. 3. As can be seen from FIG. 5,at the time of forward bias connection there occurs no snap backphenomenon that upon reaching a breakdown induced voltage, the drainvoltage is lowered. Accordingly, as shown in FIG. 6, a plurality of NMOStransistors T11 to T16 provided between an input IN and a ground levelcause an avalanche breakdown nearly uniformly when a surge voltage SV isapplied. Then, an additional surge voltage SV leads to a thermalbreakdown voltage, resulting in a thermal runaway state as in a reversebias connection.

Thus, at the time of forward bias connection, the HBM surge resistancevoltage is unchanged by changing the unit channel width resistance valueof the drain resistance of NMOS transistors. The reason for this is asfollows: Since the thermal breakdown voltage is higher than thebreakdown induced voltage, irrespective of the unit channel widthresistance value, when a plurality of NMOS transistors are connected inparallel as shown in FIG. 6, part of which cause an avalanche breakdownand also other transistors that have caused no avalanche breakdown asthe drain voltage increases, will immediately lead to an avalanchebreakdown. As a result, all the transistors cause an avalanchebreakdown, allowing the current to flow uniformly to every transistor.This results in a thermal breakdown at a thermal breakdown voltagehigher than a breakdown induced voltage.

As described, forward bias connections are by nature free from thephenomenon that the flow of current concentrates on a transistor led toan avalanche breakdown, unlike reverse bias connections. Therefore, theHBM surge resistance voltage is constant irrespective of the unitchannel width resistance value, as shown in FIG. 2.

Normally, I/O protection characteristics call for an ESD resistanceneeded in both forward and reverse bias connections. For this, a firstpreferred embodiment is directed to a semiconductor device of SOIstructure in which the ESD resistance in a reverse bias connection isincreased to be approximately equal to that in a forward biasconnection.

Construction and Operation

FIG. 7 is a diagram showing a typical plan structure of an NMOStransistor used in a reverse bias connection of an I/O protectioncircuit of the first preferred embodiment. In this figure, a drainregion 6 and a source region 7 of the NMOS transistor are represented byresistances R1 to R5 and R6 to R10, respectively. The drain region 6 andthe source region 7 can be connected to an external wiring throughcontacts 21 and 22, respectively. Merely the drain resistance value ofthe drain region 6 may be set to a predetermined value.

In cases where with the total channel width W of 400 μm and the channellength L of 0.6 μm, a plurality of NMOS transistors whose gate isgrounded are provided in parallel between an input and a ground levelsas shown in FIGS. 27 and 28, and the NMOS transistors exhibit adischarge characteristic as shown in FIG. 1, the unit channel widthresistance value of drain resistance is set to about 30 Ω.

When manufacturing resistances R1 to R10 whose unit channel widthresistance value is 30 Ω, by a silicide process, the sheet resistance of2 Ω/□(e.g., titanium silicide whose film thickness is 500 angstrom)requires 15 μm of the distance between a contact portion of a drainregion and a gate electrode (channel region), i.e., distance between agate and a contact. The sheet resistance of 10 Ω/□(e.g., cobalt silicidewhose film thickness is 400 angstrom, and titanium silicide whose filmthickness is 200 angstrom) requires 3 μm of the distance between a gateand a contact.

The reason why the unit channel width resistance value of drainresistance is set to 30 Ω is that an HBM surge resistance voltage (about1.5 KV) approximately equal to that in forward bias connections can beexpected (see FIGS. 1 and 2). By setting the unit channel widthresistance value to over 30 Ω, the HBM surge resistance voltage inreverse bias connections is greater than that in forward biasconnections. However, since it is required an ESD resistance for bothforward and reverse bias connections, even if an HBM surge resistancevoltage greater than that in forward bias connections is set at the timeof reverse bias connections, substantial improvement in I/O protectioncharacteristic cannot be attained.

In addition, as the drain voltage increases, the drive capacity of NMOStransistors decreases, reducing its operation speed. It is thereforeimpractical to increase the drain resistance than necessary.

Taking the above limit conditions into consideration, in a semiconductorof the first preferred embodiment, a drain resistance capable ofobtaining an HBM surge resistance voltage equal to that in a forwardbias is set at the time of reverse bias connections. This produces theeffect that a decrease in operability is minimized to obtain the maximumimprovement in I/O protection characteristics.

Even with a silicide protection process in which silicide is not formedpartially, a similar effect can be obtained by providing a drainresistance having the unit channel width resistance value of about 30 Ω.

In semiconductor devices formed on SOI substrates, internal circuitportions performing signal processing based on signals from an externalterminal preferably have a relatively low drain resistance in terms ofoperability, whereas I/O protection circuit portions preferably have arelatively high drain resistance as previously mentioned. Thus, whenforming NMOS transistors by providing a silicide on source and drainregions, as shown in FIG. 8, it is desirable that in an internal circuitportion 15A, a silicide layer 12 is formed relatively thick to lower thedrain resistance, and that in an I/O protection portion 15B, a silicidelayer 13 is formed relatively thin to such an extent that a drainresistance can obtain a desired HBM surge resistance voltage.

Referring to FIG. 8, side walls 11 are formed on the side faces of agate electrode 5, and silicide layer 12 (13) is formed adjacent to theside walls 11. Underneath the gate electrode 5, the side walls 11, andthe silicide layer 12 (13), a p type channel region 8, an n⁺ drainregion 6B (source region 7B), and an n⁺ drain region 6A (source region7A) are formed respectively.

A first method of forming a silicide layer 12 and a silicide layer 13 inan internal circuit portion 15A and an I/O protection circuit portion15B, respectively, so that the silicide layers 12 and 13 differ in filmthickness as shown in FIG. 8, is as follows: A drain region 6A (sourceregion 7A) of an internal circuit portion 15A and a drain region 6A(source region 7A) of an I/O protection circuit portion 15B are formedsuch that the impurity concentration of the latter is higher than thatof the former, and then a silicide layer is formed on each of the drainregion 6A (source region 7A) of the internal circuit portion 15A and theI/O protection circuit portion 15B at the same time, so that thesilicide layer 13 on the I/O protection portion 15B is thinner than thesilicide layer 12 on the internal circuit portion 15A.

The reason for this is that in the formation of a silicide layer on adrain (source) region into which impurities have been introduced withhigh concentrations, by a self-aligned silicide process, its formingspeed becomes low, and therefore, a proper manufacturing conditionspermits a thinner silicide layer formation.

A second method of forming a silicide layer 12 and a silicide layer 13in an internal circuit portion 15A and an I/O protection circuit portion15B, respectively, so that the silicide layers 12 and 13 differ in filmthickness, is as follows: A silicide layer is formed relatively thin onthe entire surface, followed by a silicide formation processing using apatterning resist in which an opening is present only above the internalcircuit portion 15A, so that only the silicide 12 of the internalcircuit portion 15A is formed thick.

Snap back phenomenon that the drain voltage is lowered upon reaching abreakdown induced voltage in reverse bias connections, is noticeablygenerated in NMOS transistors whereas it is seldom generated in PMOStransistors. The reason for this seems that in PMOS transistors holesresist moving from a source to a drain, making it difficult to occur aparasitic bipolar transistor operation. That is, the effect due to theaddition of drain resistance, which is the feature of the firstpreferred embodiment, is greater in NMOS transistors.

Therefore, a semiconductor device may be formed on an SOI substrate byadding a drain resistance only to NMOS transistors among MOS transistorsto be formed on an I/O protection circuit portion. For instance, onlysilicide layers formed on source and drain regions of NMOS transistorsin an I/O circuit may be formed thin, or no silicide layer may be formedon source and drain regions of NMOS transistors.

The Second Preferred Embodiment

The Principle

In order to improve ESD resistance, there are often employed a silicideprotection method in which no silicide is formed on an I/O protectioncircuit portion. However, if transistors formed by the silicideprotection method are operated as an I/O buffer, its operation speedbecomes low because the resistance value between a source and a drain isrelatively high.

For SOI structures, an SOI layer 3 is thin, i.e., 100 nm in filmthickness. Therefore, by an ion implantation for forming source anddrain regions, the SOI layer 3 becomes amorphous state in the directionof its entire film thickness, which is then entirely polycrystalized bythe following heat processing. This increases the drain resistancevalue, normally leading to a sheet resistance value ranging 50-1000 Ω/□.The unit channel width resistance value per 1 μm of channel width W ofMOS transistors whose channel length L is 0.3-0.5 μm, is about 1000 Ω.Accordingly, in cases where the distance from a gate to a contact, i.e.,from an external contact portion of a drain region to a channel region(gate electrode), is 1 μm and the sheet resistance is 1000 Ω/□, underthe operation at the same voltage, the current value is about one secondand the speed capability is one second that in cases where the drainresistance value is negligibly low.

In consideration of these points, a second preferred embodiment isdirected to reduce the drain resistance of MOS transistors insemiconductor devices of SOI structure that are formed by a silicideprotection.

Construction and Operation

In semiconductor devices of the second preferred embodiment, in order toreduce the drain resistance of the silicide protection portion in whichno silicide layer is formed, the amount of impurity implantation tosource and drain regions of the silicide protection portion isincreased.

FIGS. 9 and 10 are a sectional view showing a structure of asemiconductor device of the second preferred embodiment. In an internalcircuit portion 15A, an NMOS transistor is formed without silicideprotection portion, as shown in FIG. 9. In an I/O protection circuitportion 5B, an NMOS transistor is formed together with a silicideprotection layer 14, as shown in FIG. 10.

Referring to FIG. 9, side walls 11 are formed on the side faces of agate electrode 5, and a silicide layer 12 is formed adjacent to the sidewalls 11. Underneath the gate electrode 5 (gate oxide film 4), the sidewalls 11, and the silicide layer 12, a p⁻ type channel region 8, an n⁺type drain region 6B (source region 7B), and an n⁺ drain region 6A(source region 7A) are formed respectively.

Referring to FIG. 10, side walls 11 are formed on the side faces of agate electrode 5. A silicide protection layer 14 is formed adjacent tothe side walls 11 so as to cover the gate electrode 5 and the side walls11. A silicide 13 is formed adjacent to the silicide protection layer14. Underneath the gate electrode 5 (gate oxide film 4), the side walls11, the silicide protection layer 14, and the silicide layer 13, a p⁻type channel region 8, an n⁻ type drain region 6B (source region 7B), ann⁺⁺ drain region 6C (source region 7C), and an n⁺⁺ drain region 6A(source region 7A), are formed respectively.

It is noted that the impurity concentration of the drain region 6C(source region 7C) of the I/O protection circuit portion 15B is higherthan that of the drain region 6A (source region 7A) of the inter circuitportion 15A.

For instance, if the amount of impurity implantation to the drain region6A (source region 7A) of the internal circuit portion 15A is set to1-4×10¹⁵/cm², that of the drain region 6C (source region 7C) of the I/Oprotection circuit portion 15B is set to 5-20×10¹⁵/cm².

In a multicrystalized SOI layer 3, implanted impurities are trapped bygrain boundaries, it is therefore required to implant more impuritiescompared to that for a single crystal layer one. However, if formed thedrain region 6A (source region 7A) in impurity concentrations over1×10¹⁶/cm², the formation of silicide to be provided thereon will beaffected. The worst of it is that a layer of silicide is peeled off.Therefore, when forming the drain region 6C (source region 7C) inimpurity concentrations over 1×10¹⁶/cm², it is desirable that theimpurity concentration of the drain region 6A (source region 7A), uponwhich a silicide layer 13 will be formed, does not exceed 1×10¹⁶/cm²,i.e., to be lower than that of the drain region 6C (source region 7C).

The Third Preferred Embodiment

The Principle

In order that PMOS transistors have a low ESD discharge capability andhave an ESD resistance equal to that of NMOS transistors, its channelwidth is required to be greater than that of NMOS transistors. Forinstance, the channel width required for LDD structures is twice thatrequired for NMOS transistors. This necessitates much forming area,which is disadvantage. Thus, a third preferred embodiment is directed tostrike a balance between NMOS transistors and PMOS transistors used forI/O protection circuits in semiconductor device on SOI substrates, whileimproving the ESD resistance of the PMOS transistors.

The First Aspect

FIG. 12 is a plan view showing a first aspect of the third preferredembodiment. In this figure, a PMOS transistor has a gate 5P, a drainregion 6P, and a source region 7P; and an NMOS transistor has a gate 5N,a drain region 6N, and a source region 7N. As shown in FIG. 12, in orderthat the PMOS transistor has an ESD resistance equal to that of the NMOStransistor, a channel length LP of the PMOS transistor is shorter than achannel length LN of the NMOS transistor. This increases the dischargecapability of the PMOS transistor (i.e., a decrease in holding voltageor breakdown induced voltage), and also reduces its forming area.

The Second Aspect

FIGS. 13 and 14 are a sectional view showing a second aspect of thethird preferred embodiment: FIG. 13 shows a PMOS transistor formed on aninternal circuit portion 15A; and FIG. 14 shows a PMOS transistor formedon an I/O protection circuit portion 15B.

Referring to FIG. 13, side walls 11 are formed on the side faces of agate electrode 5 in the internal circuit portion 15A. Underneath thegate electrode 5 (gate oxide film 4) and the side walls 11, an n⁻ typechannel region 8 and a p⁻ type drain region 6B (source region 7B) areformed respectively. Adjacent to the drain region 6B (source region 7B),a p⁺ type drain region 6A (source region 7A) is formed.

Referring to FIG. 14, side walls 11 are formed on the side faces of agate electrode 5 in the I/O protection portion 15B. An n⁻ type channelregion 8 is formed underneath the gate electrode 5 (gate oxide film 4).Adjacent to the channel region 8B, a p⁺ type drain region 6AB (sourceregion 7AB) is formed. That is, the p type impurity concentrationbeneath the side walls 11 is set to be higher than that of the drainregion 6B (source region 7B) of the internal circuit portion 15A.

As a result, a PN junction between the drain region 6AB (source region7AB) and the channel region 8 becomes abrupt to decrease the breakdowninduced voltage, thus improving the discharge capability.

The Third Aspect

FIG. 15 is a sectional view showing a third aspect of the thirdpreferred embodiment. In the figure, a PMOS transistor formed on an I/Oprotection circuit portion 15B is shown.

Referring to FIG. 15, side walls 11 are formed on the side faces of agate electrode 5 in the I/O protection circuit portion 15B. Underneaththe gate electrode 5 (gate oxide film 4) and the side walls 11, an n⁻type channel region 8 and a p type drain region 6B (source region 7B)are formed respectively. Adjacent to the drain region 6B (source region7B), a p⁺ type drain region 6A (source region 7A) is formed. A PMOStransistor formed in an internal circuit portion 15A is the same as inFIG. 13.

As a result, a PN junction between the drain region 6B (source region7B) and the channel region 8 of the PMOS transistor in the I/Oprotection circuit portion 15B is more abrupt than a PN junction betweenthe drain region 6B (source region 7B) and the channel region 8 of thePMOS transistor in the internal circuit portion 15A. This reduces thebreakdown induced voltage to improve the discharge capability.

The Fourth Aspect

FIG. 16 is a sectional view showing a fourth aspect of the thirdpreferred embodiment. In the figure, a PMOS transistor formed on an I/Oprotection circuit portion 15B is shown.

Referring to FIG. 16, side walls 11 are formed on the side faces of agate electrode 5 in the I/O protection circuit portion 15B. Underneaththe gate electrode 5 (gate oxide film 4), an n⁻ type channel region 8 isformed. Underneath a region across portions of the gate electrode 5 andthe side walls 11A, a p⁻ type drain region 6B′ (source region 7B′) isformed. Adjacent to the drain region 6B′ (source region 7B′), a p⁺ typedrain region 6A′ (source region 7A′) is formed. A PMOS transistor formedin an internal circuit portion 15A is the same as in FIG. 13. Theformations of the drain regions 6A′, 6B′, and the source regions 7A′,7B′, are attained by an oblique ion implantation.

As a result, the channel length (effective channel length) of thechannel region 8′ of the PMOS transistor in the I/O protection circuit15B is short to improve the discharge capability of the PMOS transistor.

Other Aspects

There are other aspects as follows: by using side walls 11 of aninternal circuit portion as a mask, a p⁺ ion implantation is performedto make a graded junction due to its dispersion, thereby obtaining anI/O protection circuit portion of single drain structure, as shown inFIG. 14; an LDD structure in which the impurity concentration of a drainregion 6B (source region 7B) adjacent to a channel region 8 is high, asshown in FIG. 15; and the effective channel length is reduced by anoblique ion implantation, as shown in FIG. 16. These other aspects canalso improve the discharge capability of PMOS transistors as in thefirst to third aspect as described above.

The Fourth Preferred Embodiment

The Principle

I/O protection circuits on SOI substrates have the problem that PMOStransistors are inferior to NMOS transistors in ESD resistance (ESDdischarge capability). A fourth preferred embodiment is directed to formI/O circuits for SOI substrate by employing NMOS transistors excellentin ESD resistance.

The First Aspect

FIG. 17 is a circuit diagram showing an I/O protection circuit accordingto a first aspect of the fourth preferred embodiment. As shown in FIG.17, an input voltage IN is applied to a signal wire L1 through an I/Oterminal N1, and then transferred to an input circuit (not shown)through an inverter G1 serving as an input buffer.

An I/O protection circuit of the first aspect comprises NMOS transistorsQ11 and Q12. In the NMOS transistor Q11, a drain is connected to a powersupply, a gate is grounded, and a source is connected to the signal wireL1. In the NMOS transistor Q12, a drain is connected to the signal wireL1, and a gate and a source are grounded. That is, the I/O protectioncircuit is made up of a reverse bias connection in which the NMOStransistors Q11 and Q12 that are always in off state are connected tothe power supply side and the ground side, respectively.

Unlike devices formed on normal bulk substrates, SOI structures have nodiode between a power supply an a ground. It is therefore desired tointentionally add a diode. However, by providing an NMOS transistor witha high discharge capability therebetween, instead of a diode, it ispossible to form I/O protection circuits excellent in ESD resistancewith a high integration degree, without increasing the gate width of thetransistor.

The Second Aspect

In the construction where a power supply and a ground level areconnected in common through a power supply wire L11 and a ground wireL12, respectively, if provided an NMOS transistor between the powersupply wire L11 and the ground wire L12, either of an NMOS transistorand a PMOS transistor can be used, as an input protection transistor,between an input and the power supply wire 11, and between an input andthe ground wire L12.

With a second aspect shown in FIG. 18, even if between a power supplywire L11 and a signal wire L1, a PMOS transistor Q21 whose gate isconnected the power supply wire L11 is provided, and between the signalwire L1 and a ground wire L12, an NMOS transistor Q13 whose gate isconnected to the ground wire L12 is provided, an NMOS transistor Q14whose gate is grounded and discharge capability is high, may be providedbetween the power supply wire L11 and the ground wire L12.

With the structure in FIG. 18, even if a surge voltage is applied to aninput terminal N1, it is possible to discharge it in a ground levelthrough the signal wire L1, the PMOS transistor Q21, the power supplywire L11, the NMOS transistor Q14, and then the ground wire L12, and todischarge it in a power supply through the signal wire L1, the NMOStransistor Q13, the ground wire L12, the NMOS transistor Q14, and thenthe power supply wire L11.

Thus, providing the NMOS transistor Q14 in a diode connection betweenthe power supply wire L11 and the ground wire L12, enables to obtain I/Oprotection circuits that exhibit excellent ESD resistance on SOIsubstrates.

The Third Aspect

Referring to FIGS. 19 to 21, when an NMOS transistor is provided in adiode connection between a power supply and a ground wire L12, an inputprotection transistor may be provided either between an input and apower supply wire L11 or between the input and the ground wire L12.FIGS. 19 to 21 show a structure where an NMOS transistor Q14 whose gateis grounded is provided between the power supply wire L11 and the groundwire L12. Specifically, in the structure of FIG. 19, only an NMOStransistor Q13 whose gate is connected to a ground wire L12 is providedbetween a signal wire L1 and a ground wire L12. In the structure of FIG.20, only a PMOS transistor Q21 whose gate is connected to a power supplywire L11 is provided between a signal wire L1 and the power supply wireL11. In the structure of FIG. 21, only an NMOS transistor Q15 whose gateis connected to a ground wire L12 is provided between a signal wire L1and a power supply wire L11.

With the structure of FIG. 19, even if a surge voltage is applied to aninput terminal N1, it is possible to discharge it in a power supplythrough the signal wire L1, the NMOS transistor Q13, the ground wireL12, the NMOS transistor Q14, and then the power supply wire L11.

With the structure of FIG. 20, even if a surge voltage is applied to aninput terminal N1, it is possible to discharge it in a ground levelthrough the signal wire L1, the PMOS transistor Q21, the power supplywire L11, the NMOS transistor Q14, and then the ground wire L12.

With the structure of FIG. 21, even if a surge voltage is applied to aninput terminal N1, it is possible to discharge it in a ground levelthrough the signal wire L1, the NMOS transistor Q15, the power supplywire L11, the NMOS transistor Q14, and then the ground wire L12.

Thus, even with the structure where the input protection transistor isprovided either between the input and the power supply wire L11, orbetween the input and the ground wire L12, it is possible to discharge asurge voltage finally through the NMOS transistor Q14, leading tosatisfactorily high ESD resistance.

The Fourth Aspect

Referring to FIG. 22 showing a fourth aspect, a capacitor may beintentionally added between a power supply wire L11 and a ground wireL12. The gate capacity of a MOS transistor may be utilized as acapacitor.

In the structure of FIG. 22, an NMOS transistor Q14 whose gate isconnected to the ground wire L12 is provided between a power supply wireL11 and a ground wire L12, an NMOS transistor Q16 whose gate isconnected to the ground wire L12 is provided between a signal wire L1and the ground wire L12, and a capacitor C1 is provided between thepower supply wire L11 and the ground wire L12.

With the above structure, a surge voltage can be dispersed by making thecapacitor C1 charge the surge voltage transferred to the power supplywire L11 or the ground wire L12.

Although the fourth preferred embodiment does not refer to the structurein which a resistance (rush resistance) is provided between an input andan input protection transistor, and the structure in which an internalresistance is provided between a discharge element and an internalcircuit, these resistances can be provided to increase the ESDresistance.

Relevance between The Fourth Preferred Embodiment and The First to ThirdPreferred Embodiments

As the NMOS transistors Q11 to Q16 in the fourth preferred embodiment,the NMOS transistor of SOI structure used in the first and secondpreferred embodiments may be employed. But when employing the NMOStransistor of the first preferred embodiment, portions in which a singleNMOS transistor is formed should be replaced with a plurality of NMOStransistors in parallel connection.

As the PMOS transistors Q21 and Q22 in the fourth preferred embodiment,the PMOS transistor of SOI structure used in the first to thirdpreferred embodiments may be employed. But when employing the PMOStransistor of the first preferred embodiment, portions in which a singlePMOS transistor is formed should be replaced with a plurality of NMOStransistors in parallel connection.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing form the scope of the invention.

I claim:
 1. A semiconductor device formed on an SOI substrate, saiddevice having an I/O protection circuit portion including at least onefirst MOS transistor connected to an external terminal by a forwardbias, and a plurality of second MOS transistors connected in parallel tosaid external terminal by a reverse bias, characterized in that aresistance value of each drain resistance of said plurality of secondMOS transistors is set so that an ESD (Electro Static Discharge)resistance of said plurality of second MOS transistors is approximatelyequal to or greater than that of said at least one first MOS transistor.2. The semiconductor device of claim 1 further comprising an internalcircuit portion that performs signal processing based on signals fromsaid external terminal, said internal circuit including an MOStransistor for internal circuit having a conductivity type identicalwith that of said plurality of second MOS transistors, said MOStransistor for internal circuit having a drain resistance whoseresistance value is smaller than that of each drain resistance of saidplurality of second MOS transistors.
 3. The semiconductor device ofclaim 2, wherein a plurality of first silicide layers are respectivelyprovided on drain regions of said plurality of second MOS transistors,and a second silicide layer is provided on a drain region of said MOStransistor for internal circuit, said first silicide layers beingthinner in thickness than said second silicide layer.
 4. Thesemiconductor device of claim 1, wherein the conductivity type of saidat least one first MOS transistor and said plurality of second MOStransistors is n-type.
 5. The semiconductor device of claim 4, whereineach resistance value of said plurality of second MOS transistors is notless than 30 Ω per 1 μm of channel width.
 6. A semiconductor deviceformed on an SOI substrate, said device having an I/O protection circuitportion connecting an MOS transistor for I/O protection to an externalterminal, and an internal circuit comprising an MOS transistor forinternal circuit to perform a prescribed signal processing therein,characterized in that: a drain region of said MOS transistor for I/Oprotection has first regions upon which no silicide layer is formed, anda second region upon which a silicide layer is formed; and a drainregion of said MOS transistor for internal circuit has a third regionupon which a silicide layer is formed; said first regions being higherin impurity in concentration than said third region.
 7. Thesemiconductor device of claim 6, wherein said second region is lower inimpurity concentration than said first regions.
 8. A semiconductordevice formed on an SOI substrate, said device having an I/O protectioncircuit portion connecting PMOS and NMOS transistors for I/O protectionto an external terminal, wherein the channel region property of saidPMOS transistor is set to improve an ESD resistance of said PMOStransistor without affecting an integration degree, and wherein achannel length of said PMOS transistor is shorter than a channel lengthof said NMOS transistor.
 9. A semiconductor device formed on an SOIsubstrate, comprising: an I/O protection circuit portion connecting aPMOS transistor for I/O protection to an external terminal, the channelregion property of said PMOS transistor being set to improve an ESDresistance of said PMOS transistor without affecting an integrationdegree; and an internal circuit portion including a PMOS transistor forinternal circuit to perform signal processing therein, characterized inthat portions adjacent to a channel region in a drain region of saidPMOS transistor for I/O protection is higher in-impurity concentrationthan portions adjacent to a channel region in a drain region of saidPMOS transistor for internal circuit.
 10. The semiconductor device ofclaim 8, further comprising an internal circuit portion having a PMOStransistor for internal circuit to perform signal processing therein,characterized in that said PMOS transistor for I/O protection is shorterin channel length than said PMOS transistor for internal circuit.
 11. Asemiconductor device formed on an SOI substrate, and having an I/Oprotection circuit corresponding to an external terminal, said I/Oprotection circuit comprising: a power supply wire connected to a powersupply in common; a ground wire connected to a ground level in common; acapacitor provided between said power supply wire and said ground wire;an NMOS transistor in a diode connection between said power supply wireand said ground wire; and an MOS transistor for I/O protection providedbetween said external terminal and at least one of said power supplywire and said ground wire.
 12. A semiconductor device comprising an SOIsubstrate, a power supply wire, a ground wire, an external terminal andan I/O protection circuit connected to said external terminal, said I/Oprotection circuit having only NMOS transistors connected between saidpower supply wire and said ground wire, characterized in that said NMOStransistors include a first NMOS transistor having a grounded gateelectrode and being provided between said external terminal and saidpower supply wire, and a second NMOS transistor having a grounded gateelectrode and being provided between said external terminal and saidground wire.
 13. A semiconductor device comprising an SOI substrate, apower supply wire, a ground wire, an external terminal and an I/Oprotection circuit connected to said external terminal, said I/Oprotection circuit having only NMOS transistors connected between saidpower supply wire and said ground wire, characterized in that said powersupply wire is connected to a power supply in common, said ground wireis connected to a ground level in common, and only two of said NMOStransistors are provided which include a first NMOS transistor in adiode connection between said power supply wire and said ground wire,and a second NMOS transistor for I/O protection provided between saidexternal terminal and only one of said power supply wire and said groundwire.
 14. A semiconductor device formed on an SOI substrate, comprising:an I/O protection circuit portion including a first MOS transistorconnected to an external terminal by a forward bias when applied a surgevoltage, and a second MOS transistor connected to said external terminalby a reverse bias when applied the surge voltage; and an internalcircuit portion that performs signal processing based on signals fromsaid external circuit, said internal circuit including a third MOStransistor having a conductivity type identical to that of said secondMOS transistor; and said third MOS transistor having a drain resistancevalue smaller than that of a drain resistance of said second MOStransistor.
 15. The semiconductor device of claim 14, comprising: afirst silicide layer provided on a drain region of said second MOStransistor, and a second silicide layer provided on a drain region ofsaid third MOS transistor, wherein said first silicide layer is thinnerin thickness than said second silicide layer.
 16. The semiconductordevice of claim 15, wherein the conductivity type of said first MOStransistor and said second MOS transistor is n-type.
 17. Thesemiconductor device of claim 16, wherein a drain resistance value ofsaid second MOS transistor is not less than 30 Ω per 1 μm of channelwidth.
 18. A semiconductor device formed on an SOI substrate,comprising: an I/O protection circuit portion including a PMOStransistor connected to an external terminal and an NMOS transistorconnected to said external terminal, said PMOS transistor having achannel length shorter than a channel length of said NMOS transistor.19. A semiconductor device formed on an SOI substrate, comprising: anI/O protection circuit portion including a first PMOS transistorconnected to an external terminal; and an internal circuit portionincluding a second PMOS transistor configured to perform signalprocessing therein, said first PMOS transistor having portions of adrain region adjacent to a channel region, higher in impurityconcentration than portions of a drain region adjacent to a channelregion in said second PMOS transistor.
 20. A semiconductor device formedon an SOI substrate, comprising: an I/O protection circuit portionincluding a first PMOS transistor connected to an external terminal; andan internal circuit portion including a second PMOS transistorconfigured to perform signal processing therein, wherein said first PMOStransistor has a shorter channel length than said second PMOStransistor.